The distinctive characteristic, the great beauty of a true pearl, is its luster or orient, which is a subdued iridescence, rather than the glittering brilliance of the diamond; and unless the shelly growth be lustrous it does not rank as a gem pearl, no matter how perfect its form or beautiful its color. This luster is due to the structural arrangement of the surface as well as to the quality of the material. The nacreous material forming true pearls, and likewise mother-of-pearl, is commonly deposited in irregular tenuous layers, very thin and very small in area compared with the surface of the pearl. These laminæ overlap one another, the surfaces are microscopically crumpled and corrugated, and the edges form serrated outlines. The greater the angle which the laminæ form with the surface, the closer will be these serrated outlines, and where the plane of the exterior lamina is parallel with the plane of the surface the lines are not present. This arrangement causes the waves of light to be reflected from different levels on the surface, just as in a soap bubble, and the minute prisms split the rays up into their colored constituents, producing the chromatic or iridescent effect. The cause is wholly mechanical, and an impression of the surface made in very fine wax shows a similar iridescence. Also, if a piece of mother-of-pearl be immersed in acid until the surface lime or shelly matter is dissolved, the pellucid membrane shows the iridescence until it is so compressed that the corrugations are reduced. About two score years ago an Englishman invented steel buttons with similar minute corrugations producing pearly effect, but the manufacture was unprofitable, owing, principally, to their liability to tarnish. In the shells of some mollusks—as the edible oysters (_Ostrea_) or the giant clam (_Tridacna_),—there is almost a total absence of the crumpled corrugated laminæ, and, consequently, there is little luster. In others the nacre is of better quality, resulting in superior orient, and it probably reaches its highest degree of perfection in the pearl-oyster (_Margaritifera_). As the curvature of the surface of pearls is greater, and the minute striæ are more numerous, than in ordinary mother-of-pearl, it follows that the iridescence is likewise greater. Superior nacre is more or less translucent, depending on its quality; and to the iridescence of the outer laminæ is added that of many interior ones, so that the luster is vastly increased. The position of the pearl within the shell may greatly affect the quality of the material and, consequently, the orient. The choicest are commonly found within the soft parts of the animal, and those of poorer quality are at the edges of the mantle, or within the fibers of the adductor muscle of bivalves.